In the context of the present application, the term turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient, such as for example an intralipid solution or biological tissue. Further, light is to be understood to mean electromagnetic radiation of a wavelength in the range from 400 nm to 1400 nm. The term “optical properties” covers the reduced scattering coefficient μ′s and the absorption coefficient μa. Furthermore, “matching optical properties” is to be understood as having a similar reduced scattering coefficient μ′s and a similar absorption coefficient μa.
In recent years, several methods and devices for examining turbid media, e.g. female breast tissue, have been developed. In particular, new devices for detection and analysis of breast cancer have been developed and existing technologies have been improved. Breast cancer is one of the most occurring types of cancer: in 2002, for example, more than 1.1 million women were diagnosed and over 410,000 women died of breast cancer world-wide. Several types of devices for imaging the interior of a turbid medium by use of light have been developed. Examples for such devices are mammography devices and devices for examining other parts of human or animal bodies. A prominent example for a method for imaging the interior of a turbid medium is Diffuse Optical Tomography (DOT). In particular, such devices are intended for the localization of inhomogeneities in in vivo breast tissue of a part of a breast of a female human body. A malignant tumor is an example of such an inhomogeneity. The devices are intended to detect such inhomogeneities while they are still small, so that for example carcinoma can be detected at an early stage. A particular advantage of such devices is that the patient does not have to be exposed to the risks of examination by means of ionizing radiation, as e.g. X-rays. Furthermore, X-ray based equipment have a limited detectability and sensitivity.
U.S. Pat. No. 5,907,406 discloses a device for imaging the interior of a turbid medium by using a light source to irradiate the turbid medium and photodetectors for measuring a part of the light transported through the turbid medium. A control unit is provided for reconstructing an image of the interior of the turbid medium on the basis of the measured intensities. The disclosed device is particularly adapted for examining female breasts. In order to allow the examination of the turbid medium, the device is provided with a receptacle as a receiving volume enclosing a measuring volume and arranged to receive the turbid medium. Light from the light source is coupled into the receiving volume and into the turbid medium. The light is chosen such that it is capable of propagating through the turbid medium. For imaging an interior of a female breast, light having a wavelength within a range of 400 nm to 1400 nm is typically used. Scattered light emanating from the turbid medium as a result of coupling light into the receiving volume is coupled out of the receiving volume. Light coupled out of the receiving volume is used to reconstruct an image of an interior of the turbid medium. The light used for examining the turbid medium has to be transmitted from the light source to the turbid medium and from the turbid medium to the photodetectors. Due to different sizes of the turbid media to be examined, the size of the receptacle for receiving the turbid medium does not perfectly match the size of the turbid medium, i.e. a space remains between the receptacle and the turbid medium. The part of the turbid medium under investigation is surrounded by a matching medium filling the space in the receiving volume. The matching medium is chosen such that the optical parameters of the matching medium, such as the absorption and scattering coefficients, are substantially identical to the corresponding optical parameters of the turbid medium. In this way, image artifacts resulting from optical boundary effects that occur when light is coupled into and out of the turbid medium can be reduced. Furthermore, use of the matching medium prevents the occurrence of an optical short-circuit in the receiving volume around the turbid medium. An optical short-circuit occurs when light is detected that has propagated along a path inside the receiving volume but outside the turbid medium and, as a consequence, has not been sufficiently scattered and attenuated. In that case the intensity of the insufficiently scattered and attenuated detected light may dwarf the intensity of detected light that has been scattered and attenuated through passage through the turbid medium. The light source subsequently irradiates the turbid medium from different directions and the photodetectors measure a part of the light transmitted through the turbid medium. A plurality of such measurements are performed with the light directed to the turbid medium from different directions and, based on the results of the measurements, i.e. the obtained data set, the control unit reconstructs the image of the examined turbid medium.
In such devices, the image of the interior of the turbid medium under investigation is typically constructed by e.g. filtered backprojection or an algebraic reconstruction technique. Details on reconstruction with filtered backprojection are disclosed in “Tomographic image reconstruction from optical projections in light-diffusing media”, Appl. Optics 36, 180 (1997), for example. Information on an algebraic reconstruction technique used for optical mammography is disclosed in “First results from the Philips Optical Mammoscope” in “Photon Propagation in Tissues III”, Proc. SPIE Vol. 3194, 184 (1997), for example.
New approaches for further enhancing the accuracy of methods for detecting breast cancer by use of light have been made. For example, a fluorescent dye has been developed which can be used as a fluorescent contrast agent. For this purpose it can be injected into the body and will accumulate in cancer cells. If this fluorescent contrast agent then becomes excited with light of a suitable wavelength, the locally emitted light can be detected. Based on the emitted light, size and localization of carcinoma can be determined. Thus a powerful method for detection and localization of breast cancer is provided. In this case, for reconstructing an image of the interior of the examined turbid medium from the plurality of measurements, the spatial distribution of the fluorescent contrast agent in the interior of the turbid medium has to be reconstructed.
In known methods for reconstructing an image of the turbid medium under investigation, a reference measurement is performed before the actual measurement. In this reference measurement, the receiving volume for receiving the turbid medium during examination, having for example a cup-like shape, is completely filled with the matching medium. Then a complete reference measurement is performed in which a set of data is generated. Thereafter, the turbid medium to be examined, for example a female human breast, is placed in the receiving volume and immersed in the matching medium. The actual measurement ihoi terugs then performed in which a set of data corresponding to that of the reference measurement is generated. The set of data generated during the reference measurement is used as a reference for the set of data generated during the actual measurement. For the purpose of the reconstruction process, it is then assumed that structures inside the examined turbid medium only constitute small deviations from the homogenous matching medium, e.g. a matching fluid, which has been used during the reference measurement. Based on this assumption, in the known methods the image of the interior of the turbid medium is then reconstructed using perturbation theory with linear approximation, since the deviations from the homogenous matching medium are treated as small perturbations to the homogenous matching medium.
However, it has been found that the linear approximation is not generally valid between first measurements with only the optically matching medium and second measurements with the turbid medium immersed in the matching medium. Therefore, the reconstruction achieved using this linear approximation does not always provide satisfactory results.